1. Field of Invention
The present invention relates to an antenna and, in particular, to a planar antenna.
2. Related Art
In the recent years, the satellite communication, especially for the live shows and TV programs, is rapidly developed, and thus more than ten commercial satellites are launched every year. In additional, the satellite TV channels increase and billion users are watching these channels. Accordingly, it is very important to develop a satellite signal receiving system that can provide good quality and function. In general, the satellite signal receiving antenna is commonly designed as a dish antenna, and the LNB feed thereof usually adopts the conventional horn antenna. In order to reduce the total volume, the feed antenna can be formed on a plate circuit board so as to create a planar antenna. The planar antenna has the advantages of low manufacturing cost, less weight, suitable for mass production, and easier integration with post circuits.
FIG. 1 is a schematic diagram showing a conventional planar antenna 1, which includes a substrate 11, a patch 13, a feeding port 14, a metal ground layer 15, and a microstrip 16.
The patch 13 is a rectangular metal patch, which is formed on the upper surface of the substrate 11 by circuit printing. In addition, the metal ground layer 15 is formed on the lower surface of the substrate 11. The patch 13 is electrically connected to the feeding port 14 through the microstrip 16, so that the energy can be fed into the patch 13. Then, the length and width of the microstrip 16 can be properly adjusted to achieve the desired impedance matching of the planar antenna 1.
The planar antenna 1 can be operated in the required bandwidth by adjusting the size and shape of the patch 13. After feeding energy into the feeding port 14, the electromagnetic field can be induced between the patch 13 and the metal ground layer 15, and then the electromagnetic wave is irradiated outwardly. For receiving signals by the antenna 1, the direction of the energy transfer is converse.
Regarding to the millimeter scaled wave, a single antenna may not obtain sufficient gain, so that the antenna array composed of multiple antennas is provided to reach the desired gain. FIG. 2 is a schematic diagram showing another conventional planar antenna 1a, which includes four patches 13. The four patches 13 are the same, and the microstrip 16 electrically connects the patches 13 to the feeding port 14 so as to feed the energy into the patches 13. Then, the length and width of the microstrip 16 can be properly adjusted to achieve the desired impedance matching of the planar antenna 1a. 
The conventional planar antennas are single-polarized antennas, so they can only receive the signal from a single direction. This limits the applications of the antenna. Therefore, it is an important subject to provide an antenna that can achieve multiple polarizations, thereby increasing the utility variety.